Create structs with default parameterless constructor

Hi,

This pull request is intended to fix an issue where NLua does not create structs (or other value types) using the default, parameterless constructor. It detects when a valuetype doesn't find a constructor with zero parameters and instead uses Activator to create the object.

Thanks,
Isaac
Hi @isaacbrodsky thank you for you patch

Can you use Mono style on your patch? http://www.mono-project.com/community/contributing/coding-guidelines/
Thank you